1. Meiosis
Gamete Production

2. Occurs in specialized cells of the reproductive organs
Mitosis vs Meiosis
Occurs in all cells
Occurs in specialized cells of the reproductive organs
All new cells are identical (have the original number of chromosomes)
New cells are NOT identical  have half the original number of chromosomes
2 sets of chromosomes
(eg. humans have 2 sets of 23)
1 set of chromosomes
(sperm & egg have 1 set of 23)
Used in somatic (body) cells for growth, maintenance, repair
Used in gonads (ovaries and testicles) to produce gametes

3. Gonad (Ovary or Testes) Cells Gametes (Sperm and Eggs)
Mitosis vs. Meiosis
PMAT (mitosis)
P1M1A1T1
meiosis
P2M2A2T2
2 sets of
chromosomes
(2N)
1 set
(N)
Somatic (body) Cells
Gonad (Ovary or Testes) Cells
Gametes (Sperm and Eggs)

4. Meiosis - Terminology
Meiosis: a specialized form of cellular reproduction that is used to create reproductive cells called ‘gametes’
Gametes: reproductive cells; each contains one ‘set’ of hereditary material (chromosomes) eg. sperm/egg in animals or pollen/egg in plants
Fertilization: joining of a male (♂) gamete and a female (♀) gamete to form a ‘zygote’
Zygote: a fertilized egg

5. Meiosis - Terminology Zygote (a fertilized egg)
the first cell of the new individual
contains 2 ‘sets’ of chromosomes (2N), one set from each parent (paternal + maternal chromosomes)
Zygote
Set
Fertilization
Mitosis
etc
Male Gamete
Female Gamete

6. Meiosis - Terminology
Haploid #: Number of chromosomes in one set  also called ‘N’
Since gametes contain one set of chromosomes, they are said to be ‘haploid’
Diploid #: Number of chromosomes in two sets  also called ‘2N’
Since zygotes contain 2 sets of chromosomes, they are said to be ‘diploid’
Haploid and diploid numbers are species specific

7. Haploid and diploid numbers are species specific
For humans:
diploid number (2N) = 46 chromosomes
haploid number (N) = ____ chromosomes
For chickens:
diploid number (2N) = 78 chromosomes
For rice:
diploid number (2N) = 24 chromosomes

8. Haploid-N & Diploid-2N Male Gamete Fertilization N Mitosis 2N etc
Zygote N 2N
Fertilization
Mitosis
etc
Male Gamete
Female Gamete

9. Meiosis - Terminology Homologous chromosomes
each chromosome in the paternal set has a similar (but not identical) “partner” in the maternal set
the “partner” chromosomes have a similar physical appearance and carry genes for the same proteins
the genes (‘recipes’) for the proteins may not be identical since the chromosomes are from different individuals
maternal homologue + paternal homologue = a homologous pair of chromosomes

10. Human Chromosomes

11. Meiosis involves 2 successive cell divisions
results in the formation of 4 haploid (N) cells called gametes
in animals, the gametes are ova (eggs) and sperm
ova are produced in the ovaries in a process called oogenesis
sperm are produced in the testes in a process called spermatogenesis
occurs in 2 phases: Meiosis I and Meiosis II

12. Spermatogenesis & Oogenesis
Sperm formation
Egg formation

13. Interphase before Meiosis I
During the S phase of interphase, the DNA replicates and the identical sister chromatids stay attached at the centromere

15. Prophase I: Synapsis
the pairs of duplicated homologous chromosomes line up next to each other to form “tetrads”
the homologous chromosomes in each pair are similar to each other, but not identical:
Sister chromatids
Homologous Pair of Chromosomes
Duplicated paternal chromosome
Duplicated maternal chromosome

16. Prophase I: Crossing Over
During Prophase I, homologous chromosomes pair up in the cytoplasm to form tetrads
Areas of homologous chromosomes connect at areas called chiasmata
Crossing over happens at the chiasmata

17. Prophase I: Crossing Over
Segments of homologous chromosomes break and re-attach at similar locations
Results in new genetic combinations in the offspring
This is the main advantage of sexual reproduction

18. Prophase I: Crossing Over at Chiasmata

19. Prophase I: Results of Crossing Over
Crossing over of non-sister chromatids allows exchange of maternal and paternal genes and creates four different chromatids instead of two, increasing genetic diversity
homologous pairs of replicated chromosomes line up to form tetrads. This is called ‘synapsis’.
chromatids cross over at chiasmata
chromatids “break” and rebond at specific locations to create two new combinations of genes

20. Metaphase I
after crossing-over is complete, the homologous chromosomes remain close together as tetrads
spindle fibers line the tetrads up along the equator, with one homologous pair on each side of the equator
the tetrads arrange themselves randomly – with a mixture of paternal and maternal homologous chromosomes on each side of the equator
this random arrangement is called “random segregation” and further “mixes up” the maternal and paternal genes

21. Metaphase I
Independent assortment introduces many possible combinations of maternal and paternal genes, increasing the diversity of the offspring
For humans, there are (2)23 possible combinations.

22. Anaphase I
During Anaphase I, one half of each tetrad (one homologous pair) is pulled to each pole of the cell.
Unlike mitosis, the centromeres DO NOT BREAK, so the sister chromatids stay together
Each pole only gets half of the original chromosomes
Telophase I and cytokinesis I may or may not occur completely

23. Meiosis II
There is no interphase between Meiosis I and Meiosis II so the DNA does NOT replicate
Meiosis II follows the same steps as regular mitosis
During Anaphase II, the centromeres between the sister chromatids break and one sister chromatid is pulled to each pole of the cell
Nuclei reform and cytokinesis usually occurs (although it may be unequal).
Sister chromatids

24. Overview of Meiosis

25. Benefits of Meiosis
In mitosis (asexual reproduction), all offspring are virtually identical to the parents. If there is a change in the environment, all offspring are equally “fit” or “unfit” to cope.
In meiosis (sexual reproduction), each offspring has significantly different combinations of genes from its parents and its siblings. This genetic diversity allows some individuals to cope better with challenges and change, in the hope that some individuals will survive and keep the species from going extinct.

26. Animation of Meiosis

27. Meiosis video